Summer 2015; Variable Data Suggests Mixed Weather for the Summer

Summer as a rule can be one of the more difficult seasons to forecast for because of the lighter, variable upper wind jet dominance. This summer that general trend may not apply as much as two dominant upper wind patterns via for center stage; a relatively strong polar jet that has had no trouble visiting the Great Lakes throughout the spring along with the sub-tropical/tropical jet aided by an ever increasing El Nino.

Depending on the jet and wavelength; the El Nino aided jet stream can create a more zonal flow across the country than is typical in the summer and at times; even amplify ridging over the center of eastern part of the country as upper lows push into the West. There are actually three broad areas of warmer than normal Pacific water temperatures evident at this time (see map of sea of surface temperature anomalies in May below). The first is over the northeast Pacific extending from the Gulf of Alaska to off the Pacific Northwest. The second lies from the central Pacific extending from off the California Coast to Mexico and of course, the third El Nino itself, which is over the equator region. The second warmer region over the central Pacific was more extensive early last winter and acted as a "pseudo El Nino" and thus, was a major reason for our warmer than average December when temperatures averaged 3 1/2 to 51/2 degrees above normal..

And while your glancing at the map, note the colder than average temperatures remaining south of Greenland (off the map) that extend into northeast Canada. This is primary due to the strong Polar/Arctic jet over eastern Canada into the northern Atlantic. Also note the pockets of warmer than average SST's over the Northeast and over the Gulf.

As mentioned, the El Nino is expected to continue to strengthen into the fall as depicted below by various dynamic and statistical models. At this time last year, the models were predicting a distinct El Nino for the Winter of 2014-15 which did not materialize as the ENSO remained Neutral-borderline El Nino. This time around, the models are more aggressive with the development of El Nino and its subsequent strength. Note the SST's warming on average to between +1.5 and +2.0C

Some models are projecting a strong El Nino this fall and early winter, something not seen since 1997-98. A moderate El Nino appeared also in 2002-03 then again in 2009-10 and interestingly, both moderate El Nino winters were snowy with normal to below normal temperatures - sound familiar? Therefore; while an El Nino is projected this winter, its above normal SST's and subsequent intensity will be very important. While a strong El Nino nearly always points to a mild winter, a weak to moderate does not! But, I'm getting ahead of myself...more on the Summer of 2015!

 Year     DJF   JFM   FMA  MAM  AMJ  MJJ    JJA    JAS   ASO   SON  OND  NDJ

1997	-0.5	-0.4	-0.1	0.2	0.7	1.2	1.5	1.8	2.1	2.3	2.4	2.3
1998	2.2	1.8	1.4	0.9	0.4	-0.2	-0.7	-1.0	-1.2	-1.3	-1.4	-1.5
1999	-1.5	-1.3	-1.0	-0.9	-0.9	-1.0	-1.0	-1.1	-1.1	-1.3	-1.5	-1.7
2000	-1.7	-1.5	-1.2	-0.9	-0.8	-0.7	-0.6	-0.5	-0.6	-0.6	-0.8	-0.8
2001	-0.7	-0.6	-0.5	-0.4	-0.2	-0.1	0.0	0.0	-0.1	-0.2	-0.3	-0.3
2002	-0.2	0.0	0.1	0.3	0.5	0.7	0.8	0.8	0.9	1.2	1.3	1.3
2003	1.1	0.8	0.4	0.0	-0.2	-0.1	0.2	0.4	0.4	0.4	0.4	0.3
2004	0.3	0.2	0.1	0.1	0.2	0.3	0.5	0.7	0.8	0.7	0.7	0.7
2005	0.6	0.4	0.3	0.3	0.3	0.3	0.2	0.1	0.0	-0.2	-0.5	-0.8
2006	-0.9	-0.7	-0.5	-0.3	0.0	0.1	0.2	0.3	0.5	0.8	1.0	1.0
2007	0.7	0.3	-0.1	-0.2	-0.3	-0.3	-0.4	-0.6	-0.8	-1.1	-1.2	-1.4
2008	-1.5	-1.5	-1.2	-0.9	-0.7	-0.5	-0.3	-0.2	-0.1	-0.2	-0.5	-0.7
2009	-0.8	-0.7	-0.5	-0.2	0.2	0.4	0.5	0.6	0.8	1.1	1.4	1.6
2010	1.6	1.3	1.0	0.6	0.1	-0.4	-0.9	-1.2	-1.4	-1.5	-1.5	-1.5
2011	-1.4	-1.2	-0.9	-0.6	-0.3	-0.2	-0.2	-0.4	-0.6	-0.8	-1.0	-1.0
2012	-0.9	-0.6	-0.5	-0.3	-0.2	0.0	0.1	0.4	0.5	0.6	0.2	-0.3
2013	-0.6	-0.6	-0.4	-0.2	-0.2	-0.3	-0.3	-0.3	-0.3	-0.2	-0.3	-0.4
2014	-0.6	-0.6	-0.5	-0.1	0.1	0.1	0.0	0.0	0.2	0.5	0.7	0.7
2015	0.6	0.5	0.6

Summer 2015 Outlook for Southeast Lower Michigan

Temperatures :

Overall; I look for temperatures to average around normal but with notable swings as the upper low and troughing in eastern Canada via for dominance against upper ridging, aided and at times, even suppressed by El Nino affected winds across the south. This is a difficult forecast as we dealing with two distinctive upper wind patterns, not always present in such fashion in the summer. The outcome is not only going to be affected by conflicting air masses but also with the timing and the extent of dominance by each air mass. 

That being said; In the end, I look for  temperatures averaging between +1.5 degrees and -1.5 degrees of the summer norms across Southeast Lower Michigan...a bit wider range than typical. Analogues favor the cooler side of normal which seems reasonable considering upper wind patterns over Canada and El Nino trends. Interestingly, this temperature range may very well sum up the summer on a week or two and/or a monthly basis. 

 

Rainfall:

I look for rainfall to be quite variable as mixed data presents conflicting results and where and how much may also be exasperated more than what is typical for many summers. Taking all data (past and present) into account; rainfall is expected to be above normal over the southern sections of Southeast Lower Michigan and normal to possible even below across the Saginaw Valley and Thumb Region. As with temperatures, timing of the wettest and driest periods will be quite variable - not unlike the spring.

 

                                 Analogues for Summer 2015

The majority of these analogues were chosen due to a strengthening El Nino in play during the summer whether it turned out later either a moderate or strong winter event. That obviously wasn't going to affect the previous summer's outcome. That being said, many were chosen where the previous few seasons were either neutral or weak El Nino, as in the present case. Others were not chosen even though the summer was in a ongoing El Nino, if the previous season or two were in a La Nina Phase or the timing was off and thus, were more subjective.

 

 

As stated in the opening, the analogue summers were extremely variable but with definite trends within with four cooler than normal, six normal and two warmer with a generally a comfortable summer projected. Not surprising, the average temperature while in the normal range, leaned a bit toward below normal - makes perfect sense with twice as many cooler than warmer summers. This is not surprising as El Nino Summers lean toward the cooler side of average. Also, the chances of long hot spells are less than average and subsequently; average to below average /8-12/ so too are the amount of 90 degree days.  Overall, while a typical /six normal/ summer dominated as far as temperatures, less could be said about rainfall.

 

Rainfall was the most variable in not only seasonal amounts but areal coverage; in other words it was far less common to get close to uniform amounts. This is similar to 2013 where heavy amounts were seen over the south and lighter, below normal amounts were observed in the Saginaw Valley and Thumb. Speaking of 2013, I've noticed similarities to that summer in just this past May with the upper jet associated with mainly the Canadian upper low and to a lesser extent, the subtropical jet coming into the Southwest and South. The Summer of '13 was a Neutral Summer and thus, little affects from ENSO.

 

Summer 2013 

Stats for the Summer of '13

 

*Summer 2013	Average Temperature (Departure from Normal)	Normal
Immediate Detroit Area	71.9 / +1.1*	70.8*
Flint	70.5 / +1.1*	69.4*
Saginaw	69.7 / +0.7*	69.0*

* SE Mich Ave           70.7/+1.0                                                          69.7

Though temperatures averaged above normal the Summer of '13 it wasn't because of the highs.

Location                    June                July               August        Summer
Detroit Ave High      78.5               82.5              80.9              80.6
Normal                       79.3               83.4              81.4              81.4       
Departure                   -.8                  -.9                  -.5                 -.8

Summer 2015

Glancing at the upper wind projection for the Summer of 2013 show similarities to this summer and differences. The difference noted for this summer 2015 projection is the better subtropical jet projected and thus, a more variable upper ridge strength. This is a result of building and flattening ridging in response to short waves riding in and through the region from the west. This is depicted on both maps with the first a computer generated 500 Heights and my interpretation of active areas of surface patterns. The second map, my interpretation of summer dominant and placement of air masses.

 

Summer 2015 500H along with surface patterns

 

Summer commences June 21 at 12:38 P.M. EDT calendar-wise; while meteorologically it began June 1st and runs through August 31st.

Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian